In recent days, an RDS (radio data system) broadcasting system has come into use in European countries. A network follow system has been known as one of the services provided by the RDS broadcasting system.
In the network follow system, frequency data (AF) of a broadcasting station broadcasting a program is contained in the RDS data. AF stands for Alternative Frequencies and relates to data for indicating the frequencies of stations broadcasting the same program in circumferential areas. A receiver writes the data of the necessary AF list into a memory installed therein. When a tuning operation is carried out, the AF data is read out of the memory and signal levels of the AF frequencies are checked by a conventional method, i.e., detecting the strength of the electric field. When a signal level of a broadcasting frequency being currently received is below a preset level, the receiver checks signal levels of the AF frequencies, and selects a broadcasting frequency that can be received with better receive quality than the present one, and uses that frequency for receiving the program.
In this way, the channel select operation that will frequently occur in the mobile radio communications environment, is automatically carried out, so that a particular program can always be received in the best condition.
To take advantage of such a service, in the case of a single FM receiving system, muting is applied to a broadcasting station that is presently subjected to the audio monitor, for a short time. During this muting period, the receiver searches another broadcasting station in the same network. If a radio wave with respect to a new broadcasting station as searched can be received with better receive quality, the receiver uses the radio wave of the new station for receiving the program.
To check whether or not the searched station is contained in the same network, it takes some time. Therefore, in the case of a receiver with a single receiving system, it is inevitable that the sound reproduction is temporarily stopped.
To cope with this problem, a radio receiver provided with two receiving tuner systems is proposed.
This type of radio receiver is operable in such a way that a first tuner is used for the audio monitor, while a second tuner is used for searching the best channel. To be more exact, when a-receive quality in the first tuner is better than that in the second tuner, the receiver is instantaneously switched to the channel of the highest receiving quality. Thus, the receiver can get the service of the same network without the unnatural feelings provided to listeners based on the temporary muting periods described above.
Use of the two tuners together creates another problem to be discussed below.
Generally, a superheterodyne system is employed for these types of receivers. In the superheterodyne receiver, if a high local frequency, for example, is used for a local signal, generated by a local oscillator, a local signal of the first receiver whose frequency is the receiving frequency plus 10.7 MHz, may fall within the frequency band of the received RF signal of the second receiver. Also, the reverse case may exist i.e., a local signal of the second receiver falls within the frequency band of a received RF signal of the first receiver. This results in negatively affecting the receiving band of the other receiver.
To cope with this problem, it is conceivable that the double superheterodyne system is used for the receivers.
FIG. 3 is a block diagram showing a basic arrangement of a twin tuner of a double superheterodyne system. An RF signal received by an antenna 1 is properly amplified by RF amplifiers 2 and 3, and then applied to mixers 4 and 5. The mixers receive respectively local signals from local oscillators 6 and 7, and produce first intermediate frequency (IF) signals. The produced signals are amplified by IF signal amplifiers 8 and 9, and then applied to mixers 10 and 11, respectively. The mixers 10 and 11, which now receive the amplified IF signals, receive local signals from local oscillators 12 and 13, and convert the first IF signals into second IF signals, respectively.
The second IF frequency signals are demodulated by detector circuits 14 and 15, and output from output terminals 16 and 17, respectively.
In the receiver thus arranged, an oscillating frequency of each local oscillator 6 and 7 is selected to be at least about, for example, 20 MHz, higher than the frequency of the received RF signal. The first IF signal as produced using the first local signal is mixed with a second local signal generated by local oscillators 12 and 13 to thereby form an IF signal of 10.7 MHz. As a result, the frequency of the local signal will never fall within the frequency band of the received RF signal. Hence, it can be avoided that the local signal of the first receiver negatively affects the received RF frequency band of the second receiver.
As described above, if the radio receiver of the twin tuner type is designed so as to be free from local signal interference, the double superheterodyne system must be employed for both of the receivers.
When compared with the receiver using two single superheterodyne receiving systems, the receiver using the two double superheterodyne receiving systems needs additionally the mixers 10 and 11 and the local oscillators 12 and 13. This leads to an increase in manufacturing costs and makes the circuit arrangement complicated.